Pitch based carbon fibers have been conventionally obtained by a process comprising the steps of spinning a pitch into pitch fibers, air-oxidizing the pitch fibers at a reaction temperature of from about 200.degree. to 400.degree. C. in air to prepare infusibilized pitch fibers, and then heat-treating the infusibilized pitch fibers at a high temperature to carbonize and/or graphitize them.
However, because such infusibilization reaction by means of the air-oxidation as described above never proceeds when the reaction temperature is lower than 150.degree. C., the air-oxidation must be carried out at a high temperature.
For this reason, infusibilization techniques utilizing liquid-phase oxidation effective even at a low temperature have been developed. In these techniques, however, there is involved such a problem that the surface layer portions of the fibers are excessively oxidized to thereby readily bring about lowering of the mechanical property of the carbon fibers obtained from a pitch (see: Japanese Patent Laid-Open Publications No. 118917/1974, No. 30915/1984, No. 231825/1985, No. 502772/1986, No. 242919/1990, etc).
Japanese Patent Publication No. 42696/1973 describes a method of infusibilizing pitch fibers obtained from a specific pitch, e.g., pitch material containing high aromatic content or hydrogenation product thereof, at a temperature of not lower than about 110.degree. C. in a nitrogen oxide-containing oxidizing gas atmosphere. However, this method can be applied only to the above-mentioned specific pitch material, and any possibility of an application to such a mesophase (optically anisotropic) pitch as used in the invention is not suggested at all.
Further, in the above-mentioned Japanese Patent Publication No. 42696/1973, such a technical idea that the mesophase pitch fibers are completely infusibilized by only treating them with NO.sub.2 at a low temperature is never illustrated, though the treatment with NO.sub.2 at a low temperature described in this publication is significant as a pretreatment for the infusibilization of the pitch fibers with air.
Japanese Patent Publication No. 12740/1976 discloses a method of infusibilizing pitch fibers obtained from a fiber-forming pitch such as a petroleum pitch in a NO.sub.2 -containing oxidizing gas at a temperature of from 100.degree. to 350.degree. C. In more detail, however, according to its examples, only disclosed in this publication is a method in which pitch fibers are infusibilized at a temperature of from 100.degree. to 300.degree. C. over a period of 2.5 hours in two stages in combination with a treatment with ammonia, and this publication does not give any suggestion on a method of infusibilizing such mesophase (optically anisotropic) pitch fibers as used in the present invention over a long period of time at a low temperature of not higher than 100.degree. C. through all the process.
Japanese Patent Publication No. 24445/1992 discloses a method of infusibilizing mesophase (optically anisotropic) pitch fibers or optically isotropic pitch fibers in an oxidizing gas atmosphere containing 0.1 to 50% by volume of NO.sub.2 at a high temperature of from 150.degree. to 380.degree. C.
Namely, in the example of the publication the infusibilization treatment is carried out at a relatively high temperature such as 270.degree. to 300.degree. C. for a short period of time such as 24 to 30 minutes in a gas phase at a NO.sub.2 concentration of 2 to 5% by volume.
With respect to the infusibilization by a conventional gas-phase oxidation using air only, it is well known that the oxidation reaction does not proceed at all when the temperature is lower than 150.degree. C.
As described above, a method of infusibilizing a mesophase pitch under a NO.sub.2 -containing air is also known, however, the prior art teaches or suggests that the infusibilization of the mesophase pitch fibers by an oxidation reaction under the NO.sub.2 -containing air is not sufficiently attained, unless the infusibilization is conducted at a high temperature of not lower than 150.degree. C. The prior art also teaches that the infusibilization of a starting pitch under a NO.sub.2 -containing air at a temperature of lower than 110.degree. C. can be attained only when the specific starting pitch containing high aromatic content is used.
If the pitch fibers obtained from a mesophase (optically anisotropic) pitch are infusibilized at a high temperature, e.g., 150.degree. C., the oxidation reaction of the pitch fibers rapidly proceeds, and as a result, the pitch fibers are liable to be excessively oxidized or heterogeneously infusibilized, thereby the resultant carbon fibers do not always have homogeneous and well-balanced physical properties.